4 Conclusions

In this paper, theoretical calculations have bene carried out for the binary and ternary complexes involving the substituted benzonitrile and PO2F to study how the substituent and cooperative effect tune the π-hole pnicogen bonding interaction. In the binary complexes of R-Ph-CN…PO2F, it is found that these pnicogen bonds can be classified as medium strength interaction, possessing partially covalent characteristics, and the stability of the complexes increases with respect to substituent R in the order of CN<Br≈Cl<F<H<CH3<NH2. The electron-donating ability of the substituent adjoined with the benzene ring give rise to an increase of the interaction energy of the complexes. All the halogen atoms behave as electron-withdrawing substituents and weaken the pnicogen bond compared with Ph-CN…PO2F complexes. The geometrical analysis indicates that the dihedral angle F-P-O-O of the PO2F molecule in the complexes can well describe the strength of the intermolecular interaction. The bonding nature of the intermolecular interactions has been further explored by means of AIM and NBO methods. As for the ternary complexes, the mutual interplay between pnicogen bond and halogen bond indicated that the negative/positive synergistic effect is consistent with the geometrical changes and cooperative energy upon formation of the trimer. The effect of a pnicogen bond on a halogen bond is more pronounced than that of a halogen bond on a pnicogen bond. The results indicated that the interplay between the two interaction is strongly influenced by bonding characteristic of the bridge molecule, in addition to the strength of both interactions. When the ternary complexes contain triel bond, however, notable differences are observed for BH3 and BF3, including structural parameter and the interaction energy, which may be arising from the greater π electron density on the BF3 moiety. The MEP, AIM and NBO methodologies are used to analyze the nature of mutual interactions of the complexes. It is hoped that the results of our study are useful in a deep understanding of the nature of π-hole pnicogen bonding and how to tune it in more complex systems.